#include <bits/stdc++.h>
using namespace std;
using ll=long long;

vector<vector<int>> g;
vector<int> c;
vector<vector<int>> d;

void bfs(int v) {
    queue<int> q;
    q.push(v);
    d[v][v]=0;
    while (!q.empty()){
        int cur = q.front();
        q.pop();
        for (int to : g[cur]){
            if (d[v][to]==-1) {
                d[v][to]=d[v][cur]+1;
                q.push(to);
            }
        }
    }
}


int main(){
    int n,m;
    cin>>n>>m;
    g.resize(n+1);
    c.resize(n+1);
    d.resize(n+1, vector<int> (n+1, -1));
    for (int i=0;i<n;i++){
        cin>>c[i];
    }
    for (int i=0;i<m;i++){
        int a,b;
        cin>>a>>b;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    for (int i=0;i<n;i++) bfs(i);

    int ans=INT_MAX;
    for (int i=0;i<n;i++){
        int mn=-1;
        for (int j=0;j<n;j++){
            if (c[j]==c[i]) continue;
            if (d[i][j]==-1) continue;

            if (mn==-1 || d[i][j]<d[i][mn]) mn=j;
        }
        if (mn==-1) continue;
        int mn2=-1;
        for (int j=0;j<n;j++){
            if (c[j]==c[i] || c[j]==c[mn]) continue;
            if (d[i][j]==-1) continue;

            if (mn2==-1 || d[i][j]<d[i][mn2]) mn2=j;
        }
        if (mn2==-1) continue;
        ans=min(ans, d[i][mn]+d[i][mn2]);
    }
    if (ans==INT_MAX){
        cout<<-1;
        return 0;
    }
    cout<<ans;
}